The following is not an admission that anything discussed below is citable as prior art or part of the common general knowledge.
Polymeric separation membranes in the form of small capillary tubes or hollow fibers can be made from a variety of polymers by different methods including NIPS (non-solvent induced phase separation) and TIPS (thermally induced phase separation). Examples of NIPS processes are described in U.S. Pat. Nos. 3,615,024, 5,066,401 and 6,024,872. Examples of TIPS processes are described in U.S. Pat. Nos. 4,702,836 and 7,247,238. The membranes may have a separation layer on the inside or outside and may be used, for example, for microfiltration (MF) or ultrafiltration (UF).
A benefit of membranes in water treatment is their ability to remove bacteria from water, effectively providing physical disinfection. However, it is important to maintain mechanical integrity of the membrane for its expected service life. With hollow fiber membrane modules, one mechanical failure mechanism is fiber breakage (often near a potting interface) as a result of fatigue.
International Publication Number WO 03/097221 A1 to Yoon et al. and US Publication Number US 2002/0046970 A1 to Murase et al. describe embedding mono or multi-filament yarns longitudinally within the wall of a hollow fiber membrane as a way of reinforcing the membrane. However, upon flexing and movement of the hollow fiber, the longitudinal filaments are likely to saw through the softer membrane material and thus create a new failure mode. The inventors are not aware of any use of such a membrane in industry.
Another type of reinforced hollow fiber membrane that is currently used in industry uses a hollow textile braided sleeve coated or impregnated with a polymeric membrane. The braid provides the strength that is needed in MF/UF applications such as filtration of water suspensions or mixed liquor where continuous or intermittent agitation (with air or otherwise) of the hollow fibers is used to prevent fouling or accumulation of solids on the membrane surface.
Examples of braid-supported filtration membranes include U.S. Pat. No. 4,061,861 to Hayano et al. where a polymer is impregnated into a hollow braid to prevent shrinkage when operating at high temperature; U.S. Pat. Nos. 5,472,607 and 6,354,444 to Mahendran et al.; U.S. Pat. No. 7,267,872 to Lee et al. where the membrane is coated on the outside surface of the braid and penetration is limited; and, U.S. Pat. No. 7,306,105 to Shinada et al. where the braid is coated with two different porous layers.
Braid-supported hollow fiber membranes are normally prepared as follows. The braid is fabricated on a braider, wound on a bobbin, repackaged to larger spools by splicing ends together, and then transferred to a spin line where it is unwound and then coated or impregnated with a polymer solution in a coating head. Relatively thick walled and tightly woven braids are used so that the braid will be round-stable, meaning that it does not flatten out through winding and unwinding and is still round when inserted into the coating head.
Braided supports thus have some disadvantages. For example, round-stable braids are fabricated on braiding machines with a large number (for example 16 or more) of braiding carriers. Each carrier is supplied from a different bobbin and the bobbins must cross paths in the braiding machine. The bobbins must accelerate, decelerate and reverse radially every time the carriers cross each other. This is a costly and slow operation. Small diameter braids (less than 2 mm) are normally made at a speed of less than 0.5 m/min. In contrast, the braid coating or impregnation operation is typically done much faster, for example at a speed of greater than 15 m/min, thus the need for separate operations with a spool transfer step in between. Unwinding a large spool of braid at constant tension for membrane coating is also challenging, and the coating process must stop from time to time to change spools.
In addition, the braids used for membrane support are typically of a relatively large diameter (>1.5 mm). This is because braiding speed and braid costs are generally diameter independent, but the surface area increases proportionally with diameter. Braids thus normally have a large diameter as well as a thick wall, required to make them round-stable. As a result, the ratio of inside-to-outside diameters is small, typically smaller than 0.5. This is the normalized parameter that determines the pressure loss to conduct permeate through the lumen. High lumen pressure drop in thick wall braids limits the useful length of hollow fibers that can be potted in a module.
Fiber diameter is also a significant hidden contributor to overall membrane cost because the volume of a fiber is proportional to the square of its diameter, while the developed surface area is proportion to diameter directly. Therefore, at constant packing density of hollow fibers in a module and constant ratio of inside-to-outside diameters, an increase in the outside diameter of a fiber decreases specific surface area (area per unit volume) and increases specific polymer use (mass of polymer per unit surface area), both of which increase the cost of a membrane system designed to filter a given flow of water.